In a historically known absorption cycle, the "Generator-Absorber Heat Exchange" (GAX) cycle, heat is exchanged from the hottest section of an absorber at suction (evaporator) pressure to the coldest section of a generator at delivery (condenser) pressure. That "internal" heat exchange permits a corresponding reduction in the external heat which must be supplied to the hot section of the generator.
The GAX absorption cycle presents substantial advantages in the operating regime where the GAX effect is possible, i.e., where there is temperature overlap between part of the absorber and part of the generator. Those advantages include good efficiency (high COP); ambient-responsive efficiency (COP increases markedly as required lift decreases); very low pumping requirements (minimal parasitic losses); and reduced requirements for heat transfer surface in comparison to a comparably rated single-effect cycle.
The GAX cycle also has the advantage that it automatically defaults to an efficient single-effect cycle when the temperature overlap disappears.
Notwithstanding the above advantages, there nevertheless remain at least three shortcomings of the conventional GAX cycle. They include: 1) the heat mismatch between GAX absorber and GAX generator; 2) the loss of temperature overlap at a lift equivalent to the ideal double-effect lift; and 3) when volatile absorption working pairs are used, the need for a rectifier.
The first shortcoming derives from inherent properties of solutions. In the overlap temperature range, the generator solution is at a weaker (more diluted by refrigerant) concentration than the absorber solution. Therefore the generator solution concentration changes more rapidly with temperature, and correspondingly the heat release changes more rapidly with temperature.
Copending U.S. application Ser. No. 521994 filed on May 11, 1990 by Donald C. Erickson discloses one means of overcoming the heat mismatch shortcoming. Instead of having the same flow of absorbent component through all absorbers and generators, that application discloses increasing the flow of absorbent component through the externally heated generator and the GAX absorber, and correspondingly reducing the flow of absorbing component through the externally cooled absorber and GAX generator. That is done by incorporating a branch flow of absorbent: a second absorbent pump takes suction from between the two absorbers and discharges to between the two generators. This increases the GAX absorber heat release and decreases the GAX generator heat demand until a match is achieved over the full overlap range.
The second shortcoming, that temperature overlap only occurs at relatively low lifts, greatly reduces the usefulness of the GAX cycle. The low-lift regime where the GAX effect occurs is unfortunately the same regime where extensively developed alternative heat pumps are available, namely the mechanical compression heat pump. It would be very desirable to extend the overlap between GAX generator and GAX absorber to higher lifts, i.e., beyond the ideal double effect life. This would improve the GAX cycle efficiency in the low-lift regime where it competes against mechanical compression heat pumps. More importantly, it would also increase the GAX cycle efficiency beyond that of a single effect cycle in the higher lift regimes where mechanical compression is no longer effective. This extension of the temperature overlap range, and corresponding increase in GAX cycle COP, is one primary objective of the invention disclosed herein.
The third shortcoming of the GAX cycle is that it requires the same amount of rectification as a single effect cycle does when a volatile absorption working pair is used. In general, higher lifts cause there to be larger trace amounts of sorbent in the desorbed sorbate vapor. To prevent accumulation of liquid sorbent in the evaporator, the vapor is rectified with a reflux liquid, which causes an efficiency reduction proportional to the amount of reflux liquid used. It would be desirable to reduce or eliminate the need for rectification in all types of absorption cycles, not just in the GAX cycle. That is another objective of this disclosed invention.
One way to reduce or eliminate the efficiency penalty of rectification is to use a super-dilute absorbent. A super-dilute absorbent is one which has absorbed more sorbate than is possible to absorb at suction (evaporator) pressure and at the temperature of the externally cooled absorber. In the article "New Design of an Ammonia-Water Absorption Cooling Process . . . " by P. Vinz appearing at pages 26-34 of the December 1986 IEA Newsletter (Vol. 4 No. 4), Vinz discloses further enrichment (super-dilution) of a minor sidestream of absorbent by cooling it in a supplementary absorber to a lower temperature than the primary absorber and absorbing evaporator vapor. The enriched absorbent stream then is supplied to an auxiliary rectifier which yields a partially rectified vapor stream. That stream is partially condensed in a dephlegmator, and the resulting condensate is used to reflux the primary rectifier. Thus no external cooling is required to accomplish the rectification other than that supplied to the abnormally cool supplementary absorber.
The disadvantages of the Vinz disclosure are that it requires a source of cooling significantly colder than that supplied to the primary absorber, which is normally not available, and also that it requires numerous additional components, including the supplementary absorber and pump, the auxiliary rectifier, and the dephlegmator.
A second approach to obtaining a super-dilute absorbent is described in U.S. Pat. No. 4,311,019. In FIG. 2 of that patent Rojey et al., disclose a first cocurrent absorption of evaporator vapor at high temperature, a second externally cooled low temperature cocurrent absorption of evaporator vapor, using only part of the absorbent, and then pressurization of that fraction of the absorbent to an intermediate pressure. The pressurized partial stream of absorbent is then cocurrently desorbed by countercurrent heat exchange from the first cocurrent absorption. Next a liquid sidestream is separated from the resulting vapor-liquid mixture. Then the remaining intermediate pressure vapor-liquid mixture is liquefied in two steps by two more cocurrent absorptions. The latter (fourth) absorption is externally cooled at low temperature, and the resulting super-diluted absorbent is pumped to delivery pressure. The absorbent is then cocurrently desorbed by countercurrent heat exchange with the former (third) cocurrent absorption. The resulting vapor-liquid mixture is fed to an intermediate height of the rectification column. Rojey et al., also disclose an alternative to the liquid separation step: providing a second evaporator at higher temperature and pressure, and adding its vapor to the fluid mixture from the first cocurrent desorption rather than removing liquid. Thus the Rojey et al., disclosure for obtaining super-dilute absorbent involves three heat/mass exchanges: the absorbent to be super-diluted is first concentrated in a generator. Then diluted to the original concentration, and only then further diluted to super-dilute.
The methods disclosed by Rojey et al., to produce super-dilute absorbent present several disadvantages. First, many components or steps are required: four cocurrent absorptions; a separation or a second evaporation; two cocurrent desorptions; and two internal (GAX) heat exchanges. Secondly the first two cocurrent absorptions entail large mixing losses. Thirdly, the temperature span of the first GAX exchange is necessarily much narrower than the second, thus reducing efficiency. Finally, the manner in which the super-dilute absorbent is used, namely cocurrent desorption followed by supplying the mixture to an intermediate height of the rectification column, fails to realize any rectification reduction benefit which might otherwise be attainable.
A third prior art disclosure of a method to produce super-dilute absorbent appears in U.S. Pat. No. 4,921,515. In FIG. 2 of that patent, Dao discloses a multiplicity of generators, each operating in the same high temperature range (supplied by the same heat source) and each at a different pressure. The absorbent solution is circulated sequentially through all generators in order of decreasing pressure. Then it is circulated through a GAX absorber at suction pressure. Next it is circulated sequentially through a multiplicity of externally cooled absorbers, all at the same low temperature, and each at a different pressure. Each different pressure absorber receives vapor from the corresponding pressure generator. The absorbent is circulated through the absorbers in order of increasing pressure, i.e., a separate pump is required for each additional absorber. Each additional stage of absorption causes further super-dilution of the absorbent.
This method of super-diluting the absorbent also entails several disadvantages. Since each intermediate pressure generator is at a substantially different absorbent concentration than its associated absorber, the vapor it desorbs is at a significantly different concentration than the absorber equilibrium vapor, and hence mixing losses occur. This is exacerbated by using cocurrent mass exchange generators. Secondly, each intermediate pressure absorber demands much more vapor than the associatd generator can supply, again due to the concentration difference. For that reason, a multiplicity of additional pumps are required (87a-f) which pump the excess absorbent from each absorber up to delivery pressure. Third, high temperature heat is required to supply the vapor for each stage of super-dilution, i.e., heat that could otherwise be used to provide delivery pressure vapor in a conventional cycle. Fourth, the super-dilute absorbent is not employed in a manner which allows it to reduce or eliminate the rectification. The super-dilute absorbent is combined with less dilute absorbent streams before supply to the generator; then it is subjected to cocurrent mass exchange vice the required countercurrent; and finally the resulting vapor is combined with an extremely low quality vapor before feeding to the rectifier (66).
It has now been discovered that the availability of super-dilute absorbent for use in a generator or absorber at delivery pressure provides or makes possible four advantages. First is the increased GAX temperature overlap. Up to one third of the heat normally released from the externally cooled absorber can now be released from the GAX absorber, for delivery to the GAX desorber and resulting decrease in heat needed at the externally heated desorber. Second, with volatile absorption working pairs, the need for rectification is either eliminated or greatly reduced, because the refrigerant vapor exiting the "super-dilute" generator (or absorber) is substantially purer than that from a conventional generator. Third, a small benefit is obtained from the lower heat of desorption characteristic of more dilute absorbents--less heat is necessary to yield a given amount of refrigerant. Fourth, since the super-dilute absorbent generator operates at a colder temperature than the conventional generator, it can utilize low temperature heat which would otherwise be wasted, for example in flame-fired units it can further cool the exhaust gas.
It is noted, however, that the availability of super-dilute absorbent merely makes possible the above advantages--they will not be achieved unless appropriate enabling structure is also present.
Thus the most basic objective of this invention is a new and advantageous means of achieving a super-dilute absorbent: a means which does not require an abnormally cold source of cooling for an auxiliary absorber; a means which does not require three sequential heat/mass exchanges to produce the super-dilute absorbent; and a means which does not require the use of high-grade heat to produce the vapor required for super-dilution. Beyond that, however, the objectives of this disclosure also include recitation of enabling structure which permit the super-dilute absorbent to achieve greater GAX temperature overlap; to eliminate or reduce the required rectification; to permit increased heat match in the GAX absorber and GAX generator; and to permit ambient-responsive performance and better than single-effect performance at lifts greater that the ideal double effect lift.